Ink jetting devices are known in the art, and thus extensive description of such devices is not required herein. As described in U.S. Pat. No. 6,547,380, incorporated herein by reference, ink jet printing systems are generally of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field that adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
There are three types of drop-on-demand ink jet systems. One type of drop-on-demand system is a piezoelectric device that has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. Another type of drop-on-demand system is known as acoustic ink printing. As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. Still another type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink vehicle (usually water) in the immediate vicinity to vaporize almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands.
In a typical design of a piezoelectric ink jet device, the image is applied by jetting appropriately colored inks during four to six Notations (incremental movements) of a substrate (an image receiving member or intermediate transfer member) with respect to the ink jetting head, i.e., there is a small translation of the printhead with respect to the substrate in between each rotation. This approach simplifies the printhead design, and the small movements ensure good droplet registration. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the recording substrate, either directly or via an intermediate heated transfer belt or drum, they quickly solidify to form a predetermined pattern of solidified ink drops.
Hot melt inks typically used with ink jet printers have a wax based ink vehicle, e.g., a crystalline wax. Such solid ink jet inks provide vivid color images. In typical systems, these crystalline wax inks partially cool on an intermediate transfer member and are then pressed into the image receiving medium such as paper. Transfuse spreads the image droplet, providing a richer color and lower pile height. The low flow of the solid ink also prevents show through on the paper.
In these systems, the crystalline wax inks are jetted onto a transfer member, for example, an aluminum drum, at temperatures of approximately 130-140° C. The wax based inks are heated to such high temperatures to decrease their viscosity for efficient and proper jetting onto the transfer member. The transfer member is at approximately 60° C., so that the wax will cool sufficiently to solidify or crystallize. As the transfer member rolls over the recording medium, e.g., paper, the image comprised of wax based ink is pressed into the paper.
However, the use of crystalline waxes places limitations on the printing process. First, the printhead must be kept at about 130° C. during the print process. Moreover, when the printhead is cooled and re-warmed, the resulting contraction and expansion of the ink requires a purge cycle to achieve optimum printhead performance. Furthermore, increased mechanical robustness is desired.
While known compositions and processes are suitable for their intended purposes, a need remains for improvements in radiation curable inks, for example with respect to jetting temperatures, fusing latitude, and image quality, including low gloss image properties.
For example, conventional UV-curable inks generally provide very high gloss images. These high gloss images are very attractive in pictorial applications and on high gloss substrates. However, market experience shows that some users prefer matte or lower gloss images, such as for text and highlight color applications, and especially on low gloss substrates. UV curable inks and films tend to have high gloss values, such as about 80 ggu (Gardner Gloss Units) or higher, as measured at 75° by the GARDNER® gloss unit. However, many business applications require or prefer a lower gloss value, such as about 20 to about 50 ggu (Gardner Gloss Units). See, for example, Dalal & Swanton, TAGA Proceedings 1996, pp. 195-205 (1996).
Standard gloss reduction techniques include incorporation of particles of silica or alumina, and complex illumination schemes to selectively cure different film depths. However, such approaches are not suitable for high speed ink jet processes, as the particle size (about 25 microns) is generally too large and the resultant film is generally too thin to effectively use a dual cure scheme to generate surface irregularities and reduce gloss.